Comparative Volumetric Analysis of the Principal Brain Subdivisions in Saurian Reptiles
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- Summary [Translated in original paper.] Introduction
- Method of Study
- Delineations and Retained Architectural Units
J. Hirnforsch. 17 (1976) 513–537 Comparative Neuroanatomy Group Université Paris 7 Comparative Volumetric Analysis of the Principal Brain Subdivisions in Saurian Reptiles (by Roland Platel* with 8 Figures and 8 tables) (Received on 15 January 1976)
Summary [Translated in original paper.] Introduction After Snell (1892), it was determined that the weight of the encephalon (Pe) can be expressed as a function of body weight (Ps) by means of the formula: Pe = k • Psa. Over the years, a major effort has been made to determine the parameters α and k that pertain to each order of vertebrates from data obtained through a rigorous protocol and treated by statistical calculation. A series of previous works (Platel, 1972, 1974, 1975a) address this question for the order of saurians (and to lesser degree, for that of snakes). Thirty two species of lizards were studied and the calculation of the encephalization index for each of them enable realize a primary classifications (see Bauchot and Stephan, 1964 and 1969 for the definition and means of calculation for these data). Globally, the index of encephalization expresses the encephalic characteristics of each species (evolutionary level and/or level of adaptation); the discussion of a recent article (Platel, 1975) showed these limits and provided the rationale for a more exhaustive analysis that we will take for the first time to the level of the volume of the principal units forming the brain: telencephalon, diencephalon, brain stem, mesencephalic roof, tegmentum and medulla oblongata, cerebellum etc… For each species studied, these volumes consist of data that can be utilized in several ways:
2. It is preferable to use gross volumes and to relate each of them to the body weight of the species being considered. This procedure is an extension to the constituents of the brain of what had been previously obtained for the entire brain. Snell’s formula remains the basic relationship of such a study, nevertheless replacing brain volume (Pe) by that of the structure being analyzed (Str.) or: Str = k ∙ Psa which is better to use following logarithmic transformation: log Str. = α log Ps •|• log k. For each brain unit and with the help of samples from 32 species of saurians (mentioned in the articles already cited), such a relation leads to two categories of results: a) Calculation of the value of α and its comparison to that of the brain-body allometric coefficient of the thirty-two saurians (0.669) (Platel, 1975a) permits detecting parts of the brain that are the most “dynamic” (that is, those having a “phylogenetic” growth rate higher than that of the entire brain), those who are “stable” on the other hand (allometry close to that of the entire brain), and finally those that are “regressive” (whose “phylogenetic” growth rate is slower than that of the entire brain) (Bauchot, 1963 and 1966). b) For each species one can similarly calculate an appropriate index for each part of the brain; this index is obtained the same way as the encephalization index by taking the value derived from six lacertids as a reference. The choice of this method of calculation, which includes these “Reference Lizards” (Platel, 1975a), will be justified in the discussion. Comparison of the indices is calculated as that for the encephalization indices; it is considered as a function of adaptive or evolutionary characteristics; one could thus hope to localize the part of the brain that supports it. Nevertheless even to this level, it is frequent that some imprecisions remain; one can only hope to eradicate them by a still more thorough analysis of nuclear groupings or even cytological components of the unit in question. It is this last aspect that we will attempt to explain finally by compensating for the areas of the telencephalon (Platel, in preparation). Results and discussion refer essentially to saurians but the addition of several snakes gives ground for a tentative synthesis concerning the superorder Squamata. The study of three species of snakes also allows specifying, with a much greater acuity than that furnished by the encephalization index, that which unite and separates the saurians and snakes within the domain of brain organization.
In many cases, because we only have a single specimen, the individual being studied has been submitted as an average adult for the species considered. In other cases, one must use an animal whose brain and body weights differ from those of the reference animal. In fact, when a species sample has been studied using a significant sample, the coordinates for the average adult were obtained through calculation, and it is exceptional that they correspond to an individual from the sample. Therefore, we chose the example whose Pe and Ps are closest to the reference Pe and Ps, while giving priority to the most satisfactory Pe values. In spite of these precautions, several species remain for which it seems imperative to establish a corrective term. The volume of each measured structure would then be multiplied by the ratio of brain weight between the animal being studied and the reference animal. However, this procedure deserves some explanation. In fact, to pass the brain weight of the studied individual to the brain weight of the reference by using a simple proportion, is equal to admitting that isometry exists between one individual and another when we are aware that the intraspecific allometric coefficient Pe/Ps of saurians has a value of 0.43 (and not 1.0) (Platel, 1974). To pass the volume of a brain structure of the studied individual to the volume of the corresponding structure in the reference animal by utilizing an allometric coefficient of 0.43, commits another error of the same kind: it has been shown (Bauchot and Platel, 1971) in Scincus scincus that the allometric coefficient varies from one structure to another (for example, 0.269 for the diencephalon to 0.659 for the dorsal striatum). These two observations are the origin of the reservations that will be made later regarding the use of relative volumes. However, the establishment of a rigorous corrective measure effected through calculation whose complexity does not measure up to the results desired: it will in principle require redoing a detailed intraspecific study for each species in question analogous to the one already conducted in Scincus scincus. Unquestionably, but the study based on the latter species provides instead the opportunity to assess the errors committed when using a simple corrective measure: it is negligible (2%) relative to other errors that will be evaluated later on. Table 1 indicates with a cross (x) the species whose measures were subjected to such a correction. Method of Study The measurement of brain volume is made by applying a method whose justification and modalities have already been demonstrated (Bauchon and Platel, 1971). It is done by cutting at least 50 regularly spaced levels of the brain from front to back. As was seen in Scincus scincus, such slicing provides an approximation for the large brain subdivisions that is not improved by augmenting the number of levels. It is not the same for the smaller units which would have to be subjected to complementary measurements; in effect it is estimated that the volume of a nuclear ensemble can only be known with accuracy if this structure is represented by at least 7 or 8 levels. The cerebellum of lizards frequently has the form of a transverse lamina; also the value for this brain unit is diminished by half which yields a number of levels that varies from 6 to 28 according to the species. Some brains show long thin olfactory peduncles; in others, they are shorter and more massive; finally chameleons and snakes are devoid of them. In order that slicing is not affected by these varied aspects, calculation of the spacing is done by taking into account the slices that involve the olfactory bulbs and those things situated between the tip of the anterior olfactory nucleus (rostral part of the cerebral hemispheres) and the tip of the spinal cord (in principle up to the connection point of the first pair of cranial nerves). (Figure 1: in front of level AA on one hand, and from level BB to level CC on the other). Under these conditions the range varies from 140 μm (Hemidactylus mabouis, Psammodromus hispanicus, Anolis auratus, Chamaeleo lateralis) to 440 μm (Zonosaurus maximus) or 460 μm (Boa constrictor). Thus the following operations were performed on each series:
2. Then the limits of the architectural units were carried over to the photograms. The fragments that corresponded to each structure were cut and assembled. The parts were placed that represented the ventricles (a), optic fibers (b) as well as all the elements contributing to the initial brain weight but which would not be analyzed: meninges, choroid plexus, nerve roots, fragments of pineal gland and pituitary gland, peripheral blood vessels… (c). Lastly are made, from the use of the micrometric scale, 3 squares with the corresponding scale unit (in other words, 1 mm multiplied by the magnification). 3. The weight of these 3 squares permits calculating an average weight of a unit of area. The ratio of the weight of the elements of the same structure to the weight of the area unit provides the total surface (in mm²); the product of this by the distance separating two photograms yields the volume of the slice (in mm³). 4. The sum of the volumes of different structures, ventricles (a), optic fibers (b) and various elements (c) leads to the volume of the sliced brain (Ec). Comparing it with fresh brain weight (Pef) permits calculation of the coefficient of transformation of the sliced volumes for the corresponding fresh weight: Pef = K • Ec. This coefficient K is actually the product of the specific weight of the nerve tissue of the reptile by a coefficient K0 that expresses the volume modifications caused by the histological treatments (retraction by loss of water and lipids during several baths for fixation, dehydration, clarification, and paraffin). We cannot know precisely this specific weight, however it is known that in fishes it is: 1.0414g/cm3 (Chen, 1931) and in mammals it is 1.036 g/cm, in other words very close to 1. Consequently, it can be admitted that the error committed is negligible when using K instead of K0. Coefficient K varies from one species to another (Table II; column c. r.: coefficient of shrinkage) from 1.70 (Psammodromus hispanicus) to 2.43 (Uromastix acanthinurus): its average value is 1.99 for the 32 saurians (standard error 0.19), 2.00 for the 35 squamates (standard error 0.19). No significant error is made in considering that the brain of reptiles, like those of mammals and teleostean fishes (Ridet, Diagnk, Bauchot and Platel, 1974), is subject to reduction by half following the cutting procedure. We have nonetheless utilized the K coefficient appropriate for each specimen to calculate fresh volume. In the end, it must be admitted that the different parts of the brain undergo shrinkage with equal intensity, which is plausible, but not yet fully demonstrated.
Table 1. Presentation of studied material. List of species and abbreviations used. Columns 1 to 4: Pe (brain weight) (in mg), log Pe (–1). Ps (body weight) (in g) and log Ps of the average adult. In case there is no specimen matching these coordinates, an individual not very different should be chosen: 5. protocol number (cf. Platel, 1974 and 1975a); 6. Pe (mg); 7. log Pe (–1); 8. eventual correction (x). Fig. 1. Lizard brains, in lateral view. The vertical lines show the cross-sectional plane. The number of slices made to determine the interval of successive photograms did not take into account either portion AA-BB (olfactory peduncles) or the fragment of spinal cord situated behind segment CC. b. o.: olfactory bulbs; c: cerebellar lamina; h. c.: cerebral hemispheres; m. c.: spinal cord; n. o. a.: anterior olfactory nucleus; p. o.: olfactory peduncles; t. o.: optic tracts; T. O.: optic tectum.
2. Estimate of errors committed; search for a range of variation of gross values The object of partial volume measurement is to compare the values obtained in two different species, or between two groups of species with contrasting morphological, ecological, or phylogenetic criteria. The results derived from such a comparison only have value if they take into account the errors committed when taking measurements. This is a difficult estimation because it is the product of varying imperfections. Concomitant with this error is the biological variability of the measured volumes; an attempt will be made to express these two types of disruption by means of a global value to which the name range of variation of gross values will be given. a) The method for reconstituting and measuring brain volumes, whose principal stages were described above, was the object of an in-depth critical analysis (Bauchot, 1963). This author estimated that the delimitation of surfaces, weighing, and volume calculations are done with a global error of 5% (particularly if the nuclear structures have a roughly spherical form and if measuring their volume is done with the use of at least 4 levels). An estimate of the same order of magnitude (at least 4%) is obtained in measuring the incidence of the number of levels used for calculations of volumes in Scincus scincus: for the whole brain, taking 50 to 200 levels improves the results by reducing the total variation from 11.5% to 7.5% or 4% (Bauchot and Platel, 1971). b) A second cause of error emanates from a complex ensemble that could be referred to by the terms “studied species-observer” and which makes two factors intervene: – the inconstancy (or indeterminacy) of the observer, in the setting of limits; this is a significant obstacle that does not appears solvable for the moment. – the variability inherent to species. The analysis conducted for Scincus scincus shows that in first approximation this double disruption represents a spread of about 7.5% for the measurement values. The sum of the two variations, being 12.5% (5% ╋ 7.5%) for the gross value of measured brain volumes, is set aside for later. This percentage is in fact the average value of a variation that sometimes differs appreciably from one structure to the other; in Scincus, for example, it can reach 17% for the dorsal portion of the striatum, but it is only 9% for the diencephalon. Lastly, it can be supposed that the different species of lizards studied would behave in this regard like Scincus scincus, which is reasonable but not yet demonstrated. Preferably, comparisons are made with the help of indices rather than gross values; each index would be given with a range of variation of ± 12.5%. The comparison of the indices of two groups of species would be done by replacing the index of each of them by three values (i, 1.125 I, and 0.875 i), which allows for the introduction of measurement error and intraspecific variability in the comparison of average indices. An identical estimation performed in regard to relative volumes shows that the range of variation is only 6% in this case. This result, which singularly minimizes the measurement variation, will be addressed later; it is an additional argument for not retaining relative volumes.
Table II. Fresh volumes (in mm3) of different brain subdivisions and shrinkage coefficient values (c. r.) (transformation of sliced volumes from fresh volumes). 1. Spinal cord; a. Ventricles; b. Optic fibers; c. Various; 2. Telencephalon; 3. Diencephalon; 4. Brain stem; 5. Mesencephalic roof; 6. Tegmentum + medulla oblongata; 7. Cerebellum; E2. Brain volume with ventricles (= 2 + 3 + 4 + 7 + a). Delineations and Retained Architectural Units The description of the brain of Scincus scincus (Platel and Bachot, 1970, Bauchot and Platel, 1971) provided the opportunity to sketch the major organizational traits of a saurian brain and specify the main constituent elements. Its characteristics are those of lizards belonging to the scincomorph group, in other words that in several differences of close detail, they apply as well to species of the genus Lacerta (which we have made Reference Lizards), as to Cordylus, zonosaurs, and naturally other scincids. We have had the opportunity (Platel, 1974 and 1975) to show what might set it apart from the brain of iguanids, agamids, chamaeleonids, gekkonids, teiids, varanids, and anguids, but the main organizational lines of all these brains remain the same. The delineation of slices for the main parts of the brain rests on definitions for which it became necessary to make a choice. In fact, after almost a century, each limit has been linked to a certain number of sometimes very removed points of view. The quantified results that we present will be useful in measurement where the retained limits for different structures are included, for the moment, by the majority of authors. We have dedicated a number of descriptive works to the telencephalon of Scincus scincus (Platel, 1967, 1969, 1971); its detailed quantitative study is presently being conducted; for now the available choice is to analyze the principal constituents: the olfactory bulbs, the olfactory peduncles, and the pallium and basal areas within the cerebral hemispheres. The dorsal formations or pallium (or again episphaerium of Edinger, 1896) comprise the anterior olfactory nucleus, pars dorsalis (Platel, 1967 and 1969), paleopallium, and archipallium (Platel, 1969). The ventral formations or basal areas (or again hyposphaerium of Edinger, 1896) gather together the anterior olfactory nucleus, pars ventralis (Platel, 1967 and 1969), olfactory tubercle, strio-amygdala complex, and septum. The commissure level and the border with the rostral part of the diencephalon deserve several clarifications: the separation of the part belonging to the telencephalon (dorsally) from that representing the hypothalamus (ventrally) is made by a horizontal tract in the thickness of the anterior commissure. The lateral boundaries isolate the dorsal part of the interstitial core of the terminal stria (which will be counted with the basal areas) from the ventral part which will be counted in with the hypothalamus. Descriptions of the diencephalon group of squamates are not uncommon and address numerous species. The most recent and complete studies are those of Senn (1968): Lacerta sicula; Butler and Northcutt (1973): Iguana iguana; Cruce (1974): Tupinambis nigropunctatus; Reperant (1973): Vipera aspis. The diencephalon is subdivided into five units which were primitively recognized in reptiles by Edinger (1899), Herrick (1910), de Lange (1913), Huber and Crosby (1926), Shanklin (1930), Kuhlenbeck (1931), Papez (1935) and Arians Kappers, Huber and Crosby (1936–1960), to cite only the most in-depth studies among already established authors: epithalamus, dorsal thalamus, ventral thalamus, hypothalamus, and pretectum. It would not be a question of going beyond the measurement of volumes: the finest delineations are unusable due to the large interval imposed by slicing the entire brain into 50; a detailed study, along with more extensive slicing, will be provided at a later time (Nieuwenhuys and Platel, in preparation). Nonetheless, knowledge of the secondary components provides information that allows cutting through some contentious cases at present. On this subject we adopt the terminology and conclusions of Butler and Northcutt (1973) to which we refer for everything concerning the historical and critical approach; their nomenclature tables indicate the correspondence of terms held by the main authors in the various species studied: eight figures illustrate their description of the diencephalon of the iguana (such that we were able to find the elements on our own series of slices of Iguana iguana); they are reproduced in Figure 2 by including there, in broad terms, the boundaries of groups in which the volume was measured. The term brain stem was designated for the group of mesencephalic structures (mesencephalic tectum and tegmentum) to which the ventral part of the rhombencephalon was attached: the medulla oblongata. Because it is difficult to find the passage of the tegmentum to the medulla oblongata, we analyzed them together under the term tegmentum + medulla oblongata (Teg. + M. O.). It is possible to isolate the optic roof (optic tectum or midbrain) and posterior colliculi: the group forms the roof of the mesencephalon (or mesencephalic tectum). The cerebellum of lizards is shaped like a rather large lamina that is concave toward the front; it is reduced and lies flat on the choroid web of the fourth ventricle in snakes (Larsell, 1926 and 1967, Stefanelli, 1943, Nieuwenhuys, 1967). In all cases its delineation is easy: the cerebellar lamina (which forms the main part of the cerebellum in squamates) is taken as well as the ventral cores (medial and lateral) that attach there. The most thorough analysis of the corpus cerebelli and auricular lobes could only be considered later with the help of closer slices of this brain region. The accepted notion is that the brain ends at the level of the first pair of cranial nerves (Anthony, 1970); this convention has not always been respected during sampling of the brain, and certain species show several photograms that include the spinal cord. These measurements allow appreciating the error committed, in this case, in the estimation of brain weight; we will dedicate a paragraph to expound on the results. Fig. 2. Iguana iguana L. Main cytoarchitectural units of the diencephalon. The sketches of eight transverse levels are borrowed from Butler and Northcutt (1973). We have delineated there (in broad terms) the five diencephalic subdivisions on which our volumetric analyses are based. Epithalamus (EPI), Dorsal thalamus (THD), Ventral thalamus (THV), Pretectum (PRT), and Hypothalamus (HYPO). The other brain stages present in the same sections belong to the telencephalon (TEL) or mesencephalon – Optic roof (T. O) or tegmentum (TEG). Other abbreviations are explained in Butler and Northcutt (1973). Directory: files files -> Answer True False 2 points Question 2 files -> Integer programming and game theory files -> 4 Integer Programming files -> Bpa vehicle Window Repair Scenario #1 task: Procure vehicle window relacement. Objective files -> Pop Warner History files -> North Carolina Inclusion Initiative Mapping Where Children with ieps are Being Served Purpose files -> Fall 2013 Spring 2014 Program Data: Standard 1 Exhibit 4d files -> Hanban – asia society confucius classrooms network 2010 request for proposal files -> Northern England’s set-jetting locations Download 115.26 Kb. Share with your friends:
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